In recent years, an IC card has come into wide use, which is made of a plastic card containing such as an IC chip of a non-volatile memory, a CPU (Central Processing Unit) etc. The IC card is more advantageous in terms of handling of large amount of data, security, etc. than a magnetic card, which has been commonly used. With such advantages, much attention is now focused on an IC card as a new type card for the next generation.
The reason for rapid widespread of an IC card is compatibility with various applications. The IC card can carry out not only applications currently enforced by a magnetic card, but also applications which cannot be enforced by a magnetic card due to technical limit. Also, there has been expectation for a multi-purpose IC card capable of having plural applications in a single card.
The IC card uses a reader/writer device for power supply and writing of information. The IC card and the reader/writer device fall into contact-type and non-contact-type depending on their interfaces.
In case of contact-type, each of the IC card and the reader-writer device includes a metal connection terminal, and data exchange is performed by bringing the connection terminal of the IC card into contact with the connection terminal of the reader/writer device. Bringing the respective connection terminals into contact also enables power supply from the reader/writer device to the IC card.
In case of non-contact type, each of the IC card and the reader-writer device includes an antenna coil. The antenna coil of the reader-writer device generates a variable electric field, and the antenna coil of the IC card enters the generated variable electric field so as to allow the reader-writer device to supply power to the IC card by electromagnetic induction technology and also allow data transmission between the reader-writer device and the IC card.
The non-contact-type IC card rectifies an induced voltage generated in the antenna coil at a diode bridge before supplying the voltage to respective function blocks. Note that, in this manner, propagation between the antenna coil of the non-contact reader/writer device and the antenna coil of the non-contact IC card is carried out with an electromagnetic wave having a carrier frequency of in a range from several MHz to several 10 MHz, for example.
The non-contact IC card does not include a connection terminal for an external device, and therefore no concern is necessary for breakage of connection portion or the like. Further, the non-contact IC card is capable of power supply and data exchange with respect to the non-contact reader-writer device by only coming closer to the non-contact reader-writer device. Accordingly, the non-contact IC card is advantageous in terms of reduction of maintenance cost, easy handling, high-speed processing etc.
The non-contact reader/writer device falls roughly into two structures depending on whether or not the device has a function for fixing the non-contact IC card. The former is referred to as a close-type, while the latter is referred to as an open-type.
The communication distance between the close-type reader/writer device and IC card is not more than 5 mm, and is often 1 mm. Such a short distance of the close-type reader/writer device and IC card appeals to financial fields in which security of information plays an important role. For this reason, a conventional market has a stronger demand for a close-type reader/writer device than an open-type device.
However, in recent years, the IC card is more often used in traffic fields and also as a system for supervising entry/leaving of a room, and therefore, demand for the open-type with superior convenience increased to be greater than that for the close-type. One example can be a train ticket or a bus ticket made of an IC card, which is used by being held up by the user toward a ticket gate having an open-type reader/writer device, so as to allow the device to carry out data processing with the IC card (“hold up” data processing). Otherwise, the IC card is instantaneously brought into contact with the ticket gate (“touch and go” data processing).
However, the communication distance between the open-type reader/writer device and IC card is not fixed, thus arising a problem when power supply is carried out from the reader/writer device to the IC card. FIG. 8 shows a relation between the communication distance from an open-type reader/writer device to an IC card, and power received by the IC card, when the power supply of the open-type reader/writer device is fixed. As can be seen in the figure, the power received by the IC card becomes maximum when the communication distance is 0, and then gradually decreases as the communication distance increases.
Therefore, within an assumed range of communication distance, an IC card closer to the reader/writer device receives more power. The difference of received power is converted into heat, thus causing heat generation of the IC card when the card is placed close to the reader/writer device for a long time.
Further, to meet the expectation for a multi-purpose IC card, the power from the reader/writer device has to be increased, since the multi-purpose card requires a larger minimum voltage for driving than that of a conventional card due to its non-volatile memory of not less than 1 MB consuming great power.
FIG. 9 shows a relation between the communication distance and the received power by the IC card when more power is supplied from the open-type reader/writer device. As can be seen in the figure, the received power can be increased by supplying larger power if the communication distance between the IC card and the reader/writer device is fixed.
However, larger power supply causes an increase of power difference of the received power between respective communication distances. Further, an excessive increase of the power supply may cause an adverse effect on other devices. Thus, in view of realizing the multi-purpose IC card, it is required to reduce the heat generation caused by the received power difference.
Further, a recent demand for the IC card is a longer communication distance so as to realize superior convenience. However, to increase the communication distance, it is necessary to raise power supply from the reader/writer device as shown in FIG. 9, and therefore the same problem as above occurs.
In view of the problem of heat generation, one solution example can be found in Japanese Laid-Open Patent Application Tokukaihei 11-338983/1999 (published on Dec. 10, 1999), in which power supply from the reader/writer device is adjusted according to the communication distance between the reader/writer device and the IC card. FIG. 7 shows an arrangement example of the reader/writer device and the IC card disclosed in the foregoing publication. In the figure, the left side of the broken line shows an arrangement example of a reader/writer device 100 and the right side shows an arrangement example of an IC card 101.
The reader/writer device 100 includes an oscillator 110, a power control circuit 111, an amplifier 112, a matching circuit 113, a voltage detection circuit 114, a reader/writer IC 115, a tuning capacitor 116, and an antenna coil 117. The oscillator 110 outputs a high frequency wave, which is amplified through the amplifier 112, so as to be supplied to the antenna coil 117 via the matching circuit 113. The antenna coil 117 is directly connected to the tuning capacitor 116 so as to increase transmission efficiency. Note that, in the circuit diagram of FIG. 7, the antenna coil 117 is made up of an inductance component L1 and a resistance component R1.
The IC card 101 includes an antenna coil 120, a tuning capacitor 121, a rectification circuit section 122, and an IC card logic section 123. The tuning capacitor 121 is connected in parallel with the antenna coil 120 as with the tuning capacitor 116 connected to the reader writer device 100, so as to increase transmission efficiency sympathetically with a self-inductance L2 of the antenna coil 120. Electromagnetic coupling of the antenna coil 120 of the IC card 101 and the antenna coil 117 of the reader/writer device 100 generates an induced voltage, which is then rectified through the rectification circuit section 122 and adjusted to be a predetermined voltage before supplying to the IC card logic section 123.
The reader/writer device 100 is characterized by including a power control circuit 111 for controlling power supply to the antenna coil 117, a voltage detection circuit 114 for detecting an input voltage of the antenna coil 117, and first and second tables stored in the reader/writer IC 115.
The first table is to derive a coupling coefficient k from an output voltage Vs of the power control circuit 111 and an input voltage V to the antenna coil detected by the voltage detection circuit 114. This table is based on a formula V=Vs·f(k) (F is a function).
The second table is to derive an optimal (a voltage not causing excess heat) voltage Vs for the coupling coefficient k from the coupling coefficient k and an optimal received power P. This table is based on a formula P=g(Vs, k) (g is a function).
In the foregoing arrangement, the voltage detection circuit 114 detects an output voltage V (input voltage to the antenna coil 117) of the amplifier 112 at a certain timing. The detected value of the output voltage V is sent to the reader/writer IC 115. Since the reader/writer IC 115 controls the output voltage Vs of the power control circuit 111, it has already obtained the value of the output voltage Vs at this stage.
Next, the reader/writer IC 115 obtains the coupling coefficient k with reference to the first table by using the output voltage V of the amplifier 112 and the output voltage Vs of the power control circuit 111. Then, the reader/writer IC 115 obtains an optimal (a voltage not causing excess heat) output voltage Vs of the power control circuit 111 for the coupling coefficient k with reference to the second table by using the coupling coefficient k which has been obtained and the optimal received power P for the IC card.
Further, the reader/writer IC 115 controls the power control circuit 111 so that the power control circuit 111 outputs the obtained output voltage Vs. Consequently, it is possible to cut the excess power supply and prevent heat generation due to the power difference.
The foregoing IC card 101 and the reader/writer device 100 which can prevent heat generation due to the power difference of the received power however causes magnification of circuit scale and an increase of power consumption because of provision of the power control circuit 111 and the voltage detection circuit 114.
Further, the IC card 101 and the reader/writer device 100 still require an increase of power supply when extending the communication distance. Therefore, the problem of adverse effect to the other devices due to an increase of power supply still cannot be solved.